Tyre Myths - Wheels Magazine November 2015

[Peter M]

This month's Wheel magazine has their annual tyre comparison where they run a number of sensible buy 205/55 R 16's through a reasonably controlled testing regime.  Like previous tests the outcome is the usual "you get what you pay for" and "buy a name brand and you can't go wrong" with the most expensive tyre tested getting the overall points win.  (Yokohama Advan Sport V105 - $228) 

 

While they had the testing set up, they did some tyre myth testing that I found very interesting:

Underinflated Underperformer - I was surprised to see the relationship between tyre pressure and traction to be not as great as I expected.  In their testing dropping the tyre pressures from 32psi to 24psi - which I see as a massive change - only reduced their average cornering g's by 2% and the dry stopping difference by an additional 16cm from 80 kph. 

(This frankly blew me away as I can still recall seeing a graph plotting tyre pressures against traction in Fred Puhn's book where the graph rose very quickly and then plateaued out.  Then again it was probably 30 years ago that I last saw this book and they were probably comparing cross-plys when I think about it!)

Overinflation = Overperformer? -  Going from 32psi to 40psi resulted in a 3% increase in lateral grip and a 39cm reduction in stopping distance.  Again not as great as I expected and suggest the increase in harshness is maybe not worth it in a street car.

Wear Can Be Good Sometimes - Testing a set of normally mid runner but well worn tyres that still had 2mm tread depth allowed the posting of the best dry slalom time and among the highest cornering g's.  However what surprised me is that in the wet testing the cornering g's were only 7% less than their best performing full tread depth tyre.  The fact the worn tyres took an extra 11 m to stop over their best tyre in their wet braking testing didn't surprise me....

Here's One for You Phatt Tyre Guys - Since tyre testing was done with 205/55 16's this myth is very applicable to the front tyres of most older 911's.  I love how narrowed tyred cars steer and often can't understand why many people think that putting wide rubber on their street cars is a good thing.  If you like tramlining, kickback, higher steering effort and loss of precision, I guess it could be.  I accept there is an increase in corning grip by going wider but do you know how much?  Well, comparing same brand tyres and going up a size in width and diameter with a lower profile ratio to 225/45 17's only gave 2% more lateral grip. 

They don't comment if more of that grip increase came from the 10% extra rubber or the 10% reduction in profile.

This result was still behind their best ranked 205/55 16 in slalom times so like many things there are other greater variables at play than just size. I think I often say something similar to my wife....

 

I'm now thinking if you want more grip on the front end of your old 911, softer compound is the way to go rather than phatt's but accept for some guys its all about stance........

 

 

 

(I think Porsche thinks the same way for street driven 911's.  Recently purchased some Porsche N3 approved Michelin Pilot Sports fronts for my 993 that whilst labelled as 205/50 R17's when fitted up measure out at 195mm wide.)

 

 

 

   

 

 

 

[Peter M]

Post made me reminisce about this book: http://www.amazon.com/How-Make-Your-Car-Handle/dp/0912656468

 

[ANF]

Very true in that you do get what you pay for with tyres, just going through this now with new tyres for our fleet

Spending a bit extra for safe grip (esp wet) is no issue.

Interesting about the other "myths" explored. Maybe myth busters could test them...

(I think Porsche thinks the same way for street driven 911's.  Recently purchased some Porsche N3 approved Michelin Pilot Sports fronts for my 993 that whilst labelled as 205/50 R17's when fitted up measure out at 195mm wide.)

Did you measure the tread width or overall tyre width (sidewall to sidewall)?

I am pretty sure tyre widths are the sidewall to sidewall distance.

[Stew F]

Very true in that you do get what you pay for with tyres, just going through this now with new tyres for our fleet

Spending a bit extra for safe grip (esp wet) is no issue.

Interesting about the other "myths" explored. Maybe myth busters could test them...

Did you measure the tread width or overall tyre width (sidewall to sidewall)?

I am pretty sure tyre widths are the sidewall to sidewall distance.

I bought new 225/50-16 rears a couple of weeks ago.  They were around $225 each F&B. Bridgestone RE003.  Just booked the car in to get the fronts done. 205/55-16 RE003's @ $159 each F&B.  It seems like this spec level of tyre are all around the mark price wise.

[ANF]

I bought new 225/50-16 rears a couple of weeks ago.  They were around $225 each F&B. Bridgestone RE003.  Just booked the car in to get the fronts done. 205/55-16 RE003's @ $159 each F&B.  It seems like this spec level of tyre are all around the mark price wise.

The fronts are quite a bit cheaper!!

I went with Dunlop Direzza's for my wife's Volvo S60 - 225/45-17  these were about $178 each I think. There are very good tyres around under $200 these days

[D-banger]

Great write-up Peter and most interesting information.

I too am surprised by the small performance difference for under & over-inflation.  I still recall reading the PCV novice recommendations for a substantial over-inflation of street tyres for track days.

I am not a phatt tyre guy but am still surprised by the lack of increase in performance from a wider tyre.  I guess extra grip comes from an enlargened contact patch.  Does 10% wider tyre with 10% profile reduction increase the contact patch that much?

[ANF]

I am not a phatt tyre guy but am still surprised by the lack of increase in performance from a wider tyre.  I guess extra grip comes from an enlargened contact patch.  Does 10% wider tyre with 10% profile reduction increase the contact patch that much?

Assuming that the tread is actually 10% wider and with the overall diameter being almost the same, then 10% wider should increase contact patch by 10%

[Redracn]

Assuming that the tread is actually 10% wider and with the overall diameter being almost the same, then 10% wider should increase contact patch by 10%

But does the length of the contact patch remain the same as the tyre profile reduces? Or does it get shorter.?

[Peter M]

I too am surprised by the small performance difference for under & over-inflation.  I still recall reading the PCV novice recommendations for a substantial over-inflation of street tyres for track days.

Doug,

The recommendation by PCV to over-inflate street tyres at a track day is to reduce heat build up in the tyres so they have half a hope in lasting the day.   Twenty minutes sessions on a track for a street tyre is a bit of an ask.  So really a different environment to street driving or doing a just enough slaloms passes to get a stable result for a magazine test where the heat build up would be much less. 

I've routinely used tyre pressures somewhere between the recommended and the maximum load pressures stated on the tyre placard for decades even though I normally drive alone with no luggage thinking it gave me a substantial benefit. I have now realised that I've been fooling myself......again!  

Did you measure the tread width or overall tyre width (sidewall to sidewall)?

I am pretty sure tyre widths are the sidewall to sidewall distance.

Adam,

You are right about the definition of section width but have attached diagram just so we are all consistent with our terms:

Tyre says 205 section width on sidewall

Actual section width is 195mm

Actual tread width is 175mm

[ANF]

?But does the length of the contact patch remain the same as the tyre profile reduces? Or does it get shorter.?

Wow that is quite a bit narrower than what you would a expect a 205 "wide" tyre to be. I am off to measure the new tyres on the S60 now...

225/ 45 x 17 tyres just fitted to wife's S60 measure 220 overall and 185 tread width!

[KBGr4]

Wider really isn't always better as I found out, it has a lot to do with tyre construction and that does  (unfortunately) relate to pricing in the real world. I have ditched the 9 & 11 X 15  Gotti rims running 225 and 285 section rubber in favour of some 8  & 9 X 16's with R spec Kumho tyres for Targa High Country this year and from initial drives the car feels more in contact with the road and much more predictable. For the "period correct " look the wide rubber was and still is  great unfortunately without going to Michellin TB 15's or Pirelli Corse Classic's the grip levels were not there (particularly in the wet). As someone has suggested above the extra width does not translate into a significantly larger contact patch.

[MFX]

Great write-up Peter and most interesting information.

I too am surprised by the small performance difference for under & over-inflation.  I still recall reading the PCV novice recommendations for a substantial over-inflation of street tyres for track days.

I am not a phatt tyre guy but am still surprised by the lack of increase in performance from a wider tyre.  I guess extra grip comes from an enlargened contact patch.  Does 10% wider tyre with 10% profile reduction increase the contact patch that much?

Actually the contact patch with all things being equal with the same tyre pressure, except for a wider tyre should actually be the same surface area touching the ground. It is just a different shape. The size of the contact patch is actually more to do with tyre pressure (although there are a lot of other variables like side wall, compound, etc). But if each corner of the car weighs 600 pounds and the tyre is inflated to 30psi then the tyre will deform to a 20square inch contact patch (30 pounds per square inch).

[spiller]

I've experimented quite a lot with tyre pressures this year on track with my GT3. My experiences were that lower pressures did not result in less grip than when at optimum pressure, however it did dent the confidence significantly, as feeling the car move around on the sidewalls during high speed cornering is not nice (this is subject to tyre make as some are worse than others in this regard). By contrast, over inflating by as little as 2 psi  over the optimum hot pressures makes the car almost undrivable. This was with R-compounds and all based off bum-in-seat feeling. I do own a pyrometer and one day will test all of these theories but I usually can't be bothered pricking tyres and logging tyre temps between sessions. Checking the pressures is enough of a task after every session sometimes!

I have always been told the same thing about street tyres and over inflating when at the track. The sidewalls on street rubber are a lot softer and so too low a pressure results in lots of movement (and thus heat as Peter said). A few PSI either way on a street tyre is not near as noticeable on the track as compared with r-specs. Driving my dad's 930 at an event last weekend on ordinary street rubber, I couldn't tell the difference in the amount of grip between 38 psi hot and 33 psi, whereas its night and day in the GT3 on r-specs. There are obviously other contributing variables such as suspension setting and alignment. 

 

[symsy]

So slightly different outlook, i have a set of 996 Turbo Rims on my 993TT, I wondered if I could get your insights -  295/30/18   … Excuse me if the oppinion I invite has been covered elsewhere.

I wondered what people experience is overall with Brands

Historically , I have changed Pirelli to Michellin then last year as im in teh country and had a flat , I swapped my front in a rush and short local supply tyre from Pilot sport to Khumo and have been really impressd on my TT. I cant say I have pushed super hard in the wet , but damp and dry braking and turn in all seem fine

My real question relates to rears… again I moved from Pilot Sport 2 to a Conti Contact 2 … them seem ok but not as good potentially , cant put my finger on it but..

Any oppinions experiences… for me I always seem with the camber to get real little millage off that inner edge , even with good pressures and alignment.

With only a little difference im feeling to head back to Michellin

Some side advice was given to me to go Hankook RS3's but heard not a very comfortable ride also and Yokohama ..

Thanks

[carl888]

But does the length of the contact patch remain the same as the tyre profile reduces? Or does it get shorter.?

Assuming the same circumference, the contact patch reduces in length as a function of aspect.  That's the reason why dragsters have a VERY high profile on the rear tyres.

Also, increasing the width of a tyre doesn't give more grip.  In theory it should be about the same.  That's because as the contact patch increases, the mechanical coupling of the tyre with the road surface decreases (Because you are spreading the same weight over a larger area).  This is a massive simplification about what really happens under dynamic conditions but it makes interesting further reading. 

So slightly different outlook, i have a set of 996 Turbo Rims on my 993TT, I wondered if I could get your insights -  295/30/18   … Excuse me if the oppinion I invite has been covered elsewhere.

I wondered what people experience is overall with Brands

Historically , I have changed Pirelli to Michellin then last year as im in teh country and had a flat , I swapped my front in a rush and short local supply tyre from Pilot sport to Khumo and have been really impressd on my TT. I cant say I have pushed super hard in the wet , but damp and dry braking and turn in all seem fine

My real question relates to rears… again I moved from Pilot Sport 2 to a Conti Contact 2 … them seem ok but not as good potentially , cant put my finger on it but..

Any oppinions experiences… for me I always seem with the camber to get real little millage off that inner edge , even with good pressures and alignment.

With only a little difference im feeling to head back to Michellin

Some side advice was given to me to go Hankook RS3's but heard not a very comfortable ride also and Yokohama ..

Thanks

Are you saying you are running a different tyre front and rear on your 993 TT?

[symsy]

Assuming the same circumference, the contact patch reduces in length as a function of aspect.  That's the reason why dragsters have a VERY high profile on the rear tyres.

Also, increasing the width of a tyre doesn't give more grip.  In theory it should be about the same.  That's because as the contact patch increases, the mechanical coupling of the tyre with the road surface decreases (Because you are spreading the same weight over a larger area).  This is a massive simplification about what really happens under dynamic conditions but it makes interesting further reading. 

Are you saying you are running a different tyre front and rear on your 993 TT?

Yep - no problems with that , have mixed dunlop , with pirelli and michellin with conti others many times 

[carl888]

Yep - no problems with that , have mixed dunlop , with pirelli and michellin with conti others many times 

No offence, but that's foolish.  It mentions this in the manual.  Not to mention differing construction and TWI per manufacturer, on a 4WD car it's particularly important.   A friend did that on his 993 TT back in the day and ended up destroying the drivetrain due to the slightly different tyre sizes per manufacturer despite going with the factory sizes as per original.  It cost him $27,000 to fix in the end!  

[symsy]

No offence, but that's foolish.  It mentions this in the manual.  Not to mention differing construction and TWI per manufacturer, on a 4WD car it's particularly important.   A friend did that on his 993 TT back in the day and ended up destroying the drivetrain due to the slightly different tyre sizes per manufacturer despite going with the factory sizes as per original.  It cost him $27,000 to fix in the end!  

These Tyres will be bald before I worry about that

 

[Mike737]

I've got kumho ku-31's on the front of the 996 and they're the worst. Dry conditions they understeer a bit but are ok, in wet they feel like I'm driving on snow as it becomes more of a plough than understeer and my strategy is to trail break to get it to corner. Meanwhile the Falken 452's on the rear do a great job.

Hankook z221's do an amazing job and really bring the car to life, but they're a whole different category of tyre.

I'm switching over to a set of dunlops all round soon so will see how that goes.

Which kumhos are you running symsy? If you're looking for milage then get the tyres flipped by a tyre shop so you can wear both edges and save some money

[Peter M]

..... A friend did that on his 993 TT back in the day and ended up destroying the drivetrain due to the slightly different tyre sizes per manufacturer despite going with the factory sizes as per original.  It cost him $27,000 to fix in the end!  

Carl,

I would believe that if all roads were perfectly flat and straight but I would suggest that the differentials can cope just as well with slightly different wheel diameters as it can with curves.  I would suggest the note in the driver's manual is for other reasons.

[D-banger]

Carl,

I would believe that if all roads were perfectly flat and straight but I would suggest that the differentials can cope just as well with slightly different wheel diameters as it can with curves.  I would suggest the note in the driver's manual is for other reasons.

Pete, what "other reasons"?  Commercial?

[ByronBayChris]

No offence, but that's foolish.  It mentions this in the manual.  Not to mention differing construction and TWI per manufacturer, on a 4WD car it's particularly important.   A friend did that on his 993 TT back in the day and ended up destroying the drivetrain due to the slightly different tyre sizes per manufacturer despite going with the factory sizes as per original.  It cost him $27,000 to fix in the end!  

i was told this also on my awd wagon.  The difference in the tread thickness meant I was quoted four new tyres.    However, I suggested running the newer tyre at a slightly different psi to compensate. 

I never got an answer......anyone here ?

[Peter M]

Pete, what "other reasons"?  Commercial?

Doug, my guess would be for ABS calibration reasons but I certainly don't profess to be an expert on this topic.  

My point is that I just can't image a car manufacturer designing a powertrain that fails due to tolerance stack  across nominally correct tyre sizes.  I respectively suggest the failure was due for other reasons and the excuse of different tyre brands was not the root cause.    

[StevepGT3]

For what it's worth, my wife's Pajero, which is constant awd needed just front tyres a few years ago.Matching tyres were not available, so we got the same size in a different brand, pretty well instantly you could hear the center diff winding up so much that we had to run it in 2wd for a week until we could get new rears to match the new fronts . 

[ANF]

Part time 4wd should not be used on road as it will cause binding

http://www.4x4abc.com/4WD101/def_turnfull.html

Full time 4wd (AWD) is fine due to centre diff

http://www.4x4abc.com/4WD101/def_turnpart.html

So the centre diff in an AWD system should compensate/ allow for any difference in tyre diameter